Only three days ? ? ?
Well, drive down toward the Cape of Good Hope - one of the most scenic drives in the world, and very few cars on the road (compared to the US).
Visit Kirstenbosch Botanical Garden?
Take a hike up on Table Mountain - BUT take no valuables with you - NOR your car keys - armed robberies of tourists are very frequent.
Check out Mountain Club of S. Africa -Cape Town section -
for information on the location of recent robberies (which trailheads to avoid and which beach areas to avoid- )
If you hike the mountain, take a trail where lots of other people go, take the phone number for Mountain Rescue, and if you see unsavory guys (guys that don't look like hikers or tourists) approaching you, get away from them fast - let them rob an easier target.
Generally, if you stay out of the townships, S. Africa is quite safe. Do not take a commuter train under any circumstances.